I. PHOTODETECTORS
Unselective thermal detectors such as radiation thermocouples, bolometers and pyroelectric detectors are operated mainly at ambient temperature and can be used in an extremely wide range of electromagnetic spectrum, from x-rays to microwaves. Uncooled thermal detectors are lightweight, rugged, reliable and convenient to use. However, their detectivity is relatively low, especially when operated at high frequencies. They are used in applications which do not require high performance or fast response. The response time of thermal detectors is adequate for some imaging applications, while the wide spectral band and large number of elements can compensate for modest performance.
There are numerous applications such as fast imaging, laser receivers, real time spectroscopy and fast pyrometry, which do require high performance and fast response as well. A high sensitivity and fast response detection of infrared radiation can be achieved with the use of semiconductor photodetectors.
The major drawback of current IR photodetectors is the need for cooling to suppress thermal processes which compete with the optical ones in the generation of free carriers in a semiconductor. The capability to operate with background-limited sensitivity (BLIP) is considered the ultimate performance goal. Generally, cryogenic cooling is required to achieve this ideal situation. The cooling requirements of intrinsic narrow band gap semiconductor photodetectors are much less stringent than that of extrinsic devices, Schottky barrier and quantum well devices. Typically, to obtain background-limited performance, detectors for the 3-5 .mu.m spectral region are operated at 200 K or less, while those for the 8-14 .mu.m region are typically operated at liquid nitrogen temperature (77 K).
The conventional liquid nitrogen cooling technique is inconvenient in industry and field use. One alternative is to use Joule-Thompson microcoolers to change high-pressure nitrogen gas into a liquid directly at the cold finger of the dewar. Unfortunately, this cooling system requires a high-pressure gas tank or compressor. Another alternative is the Stirling-cycle cooler. No liquid or gas cryogen is required, only electrical power. The Stirling-cycle cooler is quite bulky, expensive and still unreliable.
The need for cooling is a considerable problem which inhibits the widespread use of IR systems. The cooling requirements add considerably to the cost, bulk, weight, power consumption and inconvenience of infrared systems and it is highly desirable that they be eliminated or reduced.